Medical system with a multi-dot electrode, where the sub-signals are combined into a synthetic reference

ABSTRACT

Medical system for detecting heart events including an electrode lead provided with a multi-dot electrode unit that comprises at least three dot electrodes, said multi-dot electrode unit is adapted to be used for intra-corporal sensing of heart signals. The heart signals sensed by each of the dot electrodes am applied to a processing means where the signals are combined and a synthetic reference signal is determined. The differences between each dot electrode heart signal and the synthetic reference voltage are determined, and an indication signal is formed based upon said differences, wherein said indication signal is used to detect heart events.

FIELD OF THE INVENTION

The present invention relates to a system according to the preamble ofthe independent claim.

BACKGROUND OF THE INVENTION

Electrode bodies adapted for sensing and/or stimulating electricalactivities of a heart and provided with a plurality of electricallyinsulated electrode surfaces at small distances from one another arepreviously known through e.g. U.S. Pat. No. 6,064,905 and U.S. Pat. No.4,848,352.

In U.S. Pat. No. 6,064,905 a multi-element tip electrode mappingcatheter is known comprising a tip section including a multi-element tipelectrode mounted at the distal end of the tip section comprising aplurality of electrode members electrically isolated from one anotherand arranged such that, during use of the catheter within the heart, atleast two different electrode members are capable of contacting theendocardium tissue at one time.

U.S. Pat. No. 4,848,352 relates to a method for cardiac pacing andsensing using combination of electrodes where a cardiac pacing lead isinserted into a heart chamber. The lead carries a plurality of separateelectrodes positioned at the distal tip of the lead and electricallyisolated from each other. A separate electrical conductor is providedfor each electrode. The electrodes may e.g. be entered in an electricalconductive relation in order to allow sensing to take place from any orall of the electrodes.

Signals detected e.g. inside a heart by a conventional heart electrodewhere the signal is detected either between two electrodes arranged at adistance from each other and close to the distal end (by using a bipolarelectrode) or between a single electrode inside the heart and anindifferent electrode arranged e.g. at the housing of the heartstimulator are often influenced by different kinds of interference.Normal interference signals may emanate from human body generatedsignals such as muscle interference or from unsuitable electromagneticfields in the patient's environment.

The interference may be reduced by the use of a bipolar electrodesystem, where a short electrode distance is one reducing factor. This isa reason for using an electrode surface having several small dotelectrodes.

However, experiments performed on animals have shown that the differencebetween the signals obtained from two dot electrodes at a multi-dotelectrode unit is difficult to detect when using detectorsconventionally used in heart stimulators. The reason is that the signalsare too similar. A conventional heart signal detector works on a pair ofelectrode surfaces and with traditional heart pacemaker signalprocessing. The traditional signal processing is amplification,band-pass filter and amplitude detection. The detector level relativethe original signal amplitude is then most often between 0.5 mV up to 10mV.

When sensing a heart signal with a lead comprising several smallelectrodes close to each other, e.g. in a multi-dot electrode unit,there may be a problem to detect the signal if conventional pacemakersignal processing is used because the differential signal may be only asmall part of the total signal amplitude. This part of the amplitude maybe in the order 0.1 to 1 mV and can often be less than 0.1 mV. Dependingon the vector of the heart activity propagation in relation to theelectrode surfaces, the differential signal can even be reduced tonearly zero. Due to the fact that the propagation vector of heartactivity varies the differential signal amplitude thereby also varies.

One way to optimise signal detection would be to monitor allcombinations of electrode pairs and momentarily select the pair withhighest amplitude or at least avoid the pair with zero amplitude. As thepropagation vector of heart activity continuously varies, the monitoringand optimisation process must be continuously ongoing which isconsidered difficult to implement.

The object of the present invention is to solve the above-stated problemregarding detection when using small electrodes arranged at a smalldistance from one another, e.g. when using a multi-dot electrode unit.

SUMMARY OF THE INVENTION

The above-mentioned object is achieved by the present inventionaccording to the independent claim.

Preferred embodiments are set forth in the dependent claims.

Thus, the object is achieved by creating a synthetic reference signalfrom all or at least three of the dot electrodes to which the sensedsignal from each of the electrodes may be related.

According to the present invention the amplitudes of all (or most of)the dot electrodes are combined to a synthetic reference signal. Theonly requirement for the signal processing is that there is somedifference between any of the dot electrode pairs. As all dot electrodesare grouped together, external interference is totally avoided and highamplification for the differential signal may be used.

The processing means that processes the detected signals may be arrangedeither close to the multi-dot electrode unit (e.g. in the electrode leadtip) or e.g. in a heart stimulator to which the electrode lead isconnected.

Thereby the signal between the synthetic reference signal and anyelectrode will have a significant amplitude, as the amplitude of theheart signal not can be totally cancelled. The requirement of theelectrodes used to create the synthetic reference signal is that not twopairs of electrodes have the same vectors.

This results in a signal separated from zero between any of theelectrodes and the synthetic reference signal, and that each signal inrelation to the synthetic reference signal will have enough amplitude tocontribute for detection and the cancellation phenomenon in bipolarsystems, i.e. two dots, is thereby avoided.

The signal processing performed by the processing means is simple andstraightforward. There is no need to filter the signals and theunsuitable side effects of the pacemaker filters (may be referred to as“memory-effects” of the filter) used today are avoided.

The present invention accomplishes total avoidance of some of the signalproblems in pacemakers as signal detection of different types ofinterference such as external interference or far field heart signals.

In a preferred embodiment of the present invention an electrode lead isprovided with the multi-dot electrode unit and a processing meansarranged close to the multi-dot electrode unit and supplies detectionsignal(s) from the processing means through the lead into the heartstimulator.

SHORT DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 shows a block diagram of the medical system according to apreferred embodiment of the present invention.

FIG. 2 shows a block diagram that schematically illustrates themulti-dot electrode unit and the processing means according to thepreferred embodiment.

FIG. 3 shows a diagram of a preferred embodiment as a schematic ofelectronics of the present invention.

FIG. 4 is a cut through view of a multi-dot electrode unit in the distaltip of an electrode lead.

FIG. 5 is a cross-sectional view of the multi-dot electrode unit in thedistal tip of an electrode lead as shown in FIG. 4.

FIG. 6 shows signal recordings from seven dot electrodes of a multi-dotelectrode unit placed in the ventricle of an animal.

FIG. 7 shows a small part of the signal recordings as in FIG. 6 in alarger scale.

FIG. 8 shows differences of signal recordings of seven dot electrodes(S1-S7) of a multi-dot electrode unit where electrode S1 is used as areference.

FIG. 9 shows differences of signal recordings of seven dot electrodes(S1-S7) of a multi-dot electrode unit where the synthetic referencesignal (SR) calculated according to the present invention is used as areference.

FIG. 10 shows three signal tracings of different processed signals.

FIG. 11 shows the same signals as in FIG. 10 but with another timescale.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A multi-dot electrode unit used in connection with the present inventionis preferably arranged at the distal end of an electrode lead. Themulti-dot electrode unit includes a plurality of dot electrodeselectrically insulated from one another and arranged at small distancesfrom one another. An exemplary distance between dot electrodes is lessthan 1 mm.

In its broadest aspect a dot electrode should be regarded as a smallelectrode surface having an optional geometric shape, e.g. circular,elliptical, rectangular, square, band-shaped etc. In case of a circulardot electrode the diameter is in the interval of 0.05-1 mm.

The distance between adjacent dot electrode surfaces is preferably lessthan 1 mm. The distance between the most distant dot electrodes in amulti-dot electrode unit is less than 10 mm.

The electrode lead provided with the multi-dot electrode unit will beable to pick up nearly the same electrical signal on each dot electrode.

In order to perform bipolar sensing using the dot electrodes thedifferential signal (that is the signal between two dot electrodes) issmall and a special effort must be taken in the design to avoidso-called common mode effects.

Initially a general overview of the present invention will be given.

According to the present invention the signals from N dot electrodes areadded together in order to create an un-normalized synthetic referencesignal. N could be any number between 3 and the total number of dotelectrodes. The total number of dot electrodes may be very large but asuitable number is between 10 and 30, preferably 20. In one embodiment Nis in the range 4-8. The created signal is then divided by N and anormalized synthetic reference signal is obtained. The differentialsignal to be used for detection is made in relation to the normalizedsynthetic reference signal.

The following example further illustrates how the synthetic referencesignal is created and how a differential signal for a specific dotelectrode in relation to the created synthetic reference signal isdetermined.

The signal amplitude at a dot electrode “i” is designated U_(i) inrelation to a reference. This reference may be obtained from a pointcapable of providing a long-term stable reference level, e.g. a separateindifferent electrode at the pacemaker housing or the battery in thepacemaker. The un-normalized synthetic reference signal will then be:Σ(U₁+ . . . +U_(N))

A normalized synthetic reference signal (SR-signal) will then be:SR-signal=(1/N)×Σ(U ₁ + . . . +U _(N))

The differential signal A_(diff(i)) for dot electrode i will then be:A _(diff(i)) =U _(i)−(1/N)×Σ(U ₁ + . . . +U _(N))

This differential signal is amplified to a suitable detecting level. Asthe short distance between the dot electrodes essentially reduces theinfluence of interference the only filtering necessary to perform is totransfer the signal shape to a shape that is easy to detect.

To detect the heart signal from activity near the dot electrodes thecontribution from all electrodes is most optimal. As the differencesignal of each dot electrode relative the synthetic reference can beboth positive and negative the difference signals must be processedprior the signals are combined.

According to a preferred embodiment of the present invention theabsolute values of the difference signals are determined and then addedtogether to form an indication signal. This sign conversion may beachieved by rectification. Alternatively and according to an alternativeembodiment, the difference signals instead are squared and then addedtogether to form the indict signal. Still a further possibility is toform a value representing the signal contents for the difference signalsupon which the indication signal is formed.

According to a preferred embodiment of the present invention the signalprocessing means is arranged close to the multi-dot electrode unit. Asindicated above the multi-dot electrode unit preferably is placed at thedistal tip of the electrode lead. However, any position along theelectrode lead may be possible without departing from the scope of thepresent invention as it is defined in the appended claims.

Thus, according to this preferred embodiment the processing meansincludes a specific amplifier and detector circuit placed close to theelectrode tip. In this case a fully integrated circuitry with enoughsmall size to be placed within one or a few cubic millimetres isarranged in the electrode tip.

With regard to the power supply of the processing means two conductorsare arranged in the electrode lead from the heart stimulator (thatincludes a battery) to the electrode tip. One of the conductors may bethe one used for heart stimulation. The information from the signaldetection may also be sent from the electrode tip to the heartstimulator on the same pair of conductors. By using switched loading analtered voltage level may transfer information in binary format from theelectrode tip into the heart pacemaker. The signal transfer does notcontain complete information with regard to full signal morphology butonly detection information is transferred. The detection information maythen be used by control means in the heart stimulator to control e.g.the pulse generation.

The signal processing performed by the processing means to detect heartactivity may be done in several ways, for example band pass filtering ofthe signals, adding signals, correlate signals together etc.

The type of electronics can be of several types, e.g. traditionalanalogue, analogue to digital converted and then digitally processed,analogue sampled and processed in analogue way by a so-called switchcapacitor technique. Thus, the present invention may be implemented inmany different ways where some will be described in the following.

The preferred implementation of the processing means in view oftechnical realisation as micro-power electronics with smallestdimensions to be assembled into the electrode tip is the so-calledsampled switch cap technique.

However, for easier understanding of this implementation the descriptionis as if it were traditional analogue technique keeping in mind thatthere is methods for direct translation between those techniques.

FIG. 1 shows a block diagram of the medical system according to apreferred embodiment of the present invention. The system comprises anelectrode lead 2 provided with a multi-dot electrode unit 4 including aprocessing means 6. The multi-dot electrode unit 4 is provided with aplurality of dot electrodes 8 and may be arranged near or in contactwith a heart wall, e.g. in the right ventricle of the heart.

In the figure the multi-dot electrode unit is arranged close to thedistal end of the electrode lead, i.e. in the electrode lead tip, butmay also be arranged at another position along the lead, e.g. within theform of a cylindrical ring electrode at a predetermined distance fromthe distal end.

The electrode lead 2 is coupled to an implantable heart stimulator 10that inter alia comprises heart therapy and control means 12 and powersupply means 14.

FIG. 2 shows a block diagram that schematically illustrates themulti-dot electrode unit and the processing means according to thepreferred embodiment. Each of the dot electrodes 8 ₁-8 _(N) is connectedto a respective amplifying means 16 ₁-16 _(N) in order to amplify thedot electrode signals sensed by the dot electrodes.

The signal (U₁-U_(N)) provided to each amplifier 16 must be in relationto any reference voltage. The reference voltage could be any stablevoltage potential such as the power supply voltage or a derivate of suchvoltage.

If the amplitude of a sensed dot electrode signal is in the order of0.1-2 mV the amplification should be in the range of 50-2000 in order toobtain a dynamic detection voltage in the order of 1 Volt. Theamplifying means are connected to a synthetic reference calculationmeans 18 where a synthetic reference signal (SR-signal) is calculated.

The synthetic reference signal and the amplified dot electrode signalsare applied to difference signal means 20 ₁-20 _(N) where differencesbetween each of the amplified dot electrode signals and the syntheticreference signal are formed according to the formulaA_(diff(i))=U_(i)−SR-signal, where i=1 . . . N.

According to a preferred embodiment of the present invention thedifference signals are applied to rectifying means 22 ₁-22 _(N) wherethe signals are rectified and these rectified signals are in turnapplied to a summing means 24. The summing means determines anindication signal that is applied to a detector means 26 provided withpredetermined heart event detection criteria that may include one ormany threshold(s) representing different predetermined heart events tobe detected. One or many detection signals is(are) generated by thedetector means as a result of the these comparisons.

The multi-dot electrode unit is in particular suitable for detecting“fast” heart events in the close vicinity of the electrode. If themulti-dot electrode unit is placed in the atrium these fast heart eventscould be spontaneous or stimulated P-waves and atrial fibrillation orflutter and if the electrode is placed in the ventricle spontaneous orstimulated R-waves could be detected.

According to an alternative embodiment of the present invention theprocessing means is arranged at a distance from the multi-dot electrodeunit, e.g. inside the body in an implantable heart stimulator or outsidethe body in an external medical device. In this alternative embodimenteach dot electrode is connected via a separate conductor in theelectrode lead to the processing means located e.g. inside a heartstimulator housing.

FIG. 3 shows a diagram as a schematic of electronics illustrating thecircuitry of the preferred embodiment of the present invention. In theillustrated embodiment three of the N dot electrodes are shown. Therectangles represent resistors having the resistance R except whereindicated a resistor value of R/N and the triangles representoperational amplifiers.

U₁-U_(N) are the dot electrode potentials in relation to an electricalreference point.

In the figure U_(G) represents a ground potential against which each ofthe dot electrode potentials are related to. It should be noted thatU_(G) is a conducting surface (e.g. a metal surface) close to themulti-dot electrodes (e.g. inside said processing means). This potential(U_(G)) is not stable compared to e.g. a dot electrode since it isrelated to the battery source and the housing of the implant. However,if this defined ground potential (U_(G)) or zero potential is usedinternally in the processing means as the earth potential then noadditional disturbance or interference is induced during the furtherprocessing of the signals.

Using the same reference signs as in FIG. 2 the circuitry in FIG. 3includes the amplifying means 16 that amplify the sensed heart signalsand generate dot electrode signals (U₁-U_(N)), the synthetic referencecalculation means 18 that calculates the synthetic reference signal(SR-signal) and the difference signal means 20 that determines thedifference signals A_(diff(1))-A_(diff(N)).

FIG. 4 is a cut through view of a multi-dot electrode unit in the distaltip of an electrode lead with processing means 6 and two conductors 32connecting the processing means to the control means in the pacemaker.The distal tip is a hermetic capsule of ceramic material with a cavity34 housing the processing means. The diameter of the capsule is in theorder of 3 mm. The two conductors pass through the wall of the capsulewhere it is connected to the processing means via wire bonding 36. Thedot electrodes 8 are evenly spread on the end of the electrode tip andare connected to the processing means.

FIG. 5 is a cross-sectional view of the multi-dot electrode unit in thedistal tip of an electrode lead as shown in FIG. 4. The cavity 34 isclosed with a lid 38 inserted from above and hermetically sealed to thecapsule. The sealing may consist of a low temperature melting glass orsome type of metallic soldering.

FIG. 6 shows signal recordings from seven dot electrodes of a multi-dotelectrode unit placed in the ventricle of an animal where the R-waveseasily may be identified. The signals are unipolar which means that theyare measured between each of the electrodes and an electrode surface atthe capsule of the pacemaker. The time scale (horizontal axis) is from 0to 3 seconds. The signal from electrode 7 is at the top and electrode 1at the bottom. Only small non-obvious differences between the electrodesignals may be identified.

FIG. 7 shows a small part of the signal recordings as in FIG. 6 in alarger scale (0.105-0.130 seconds). Also here only small differences maybe identified.

FIG. 8 shows differences of signal recordings of seven dot electrodes(S1-S7) of a multi-dot electrode unit where electrode S1 is used as areference. The time scale is 0.105-0.130 seconds. The signal S6-S1 iscancelled out because the signals S6 and S1 are too similar.Investigations have shown as a rule that one or two of the bipolarsignals are cancelled out. Which pair that is cancelled out cannot bepredicted. Even if one pair is good and another is bad the situation maybe opposite duets varying conditions such as varying propagation vectoror activity. This illustrates that the use of one bipolar signal fromthe multi-dot electrode unit is not sufficient and if more bipolarsignals than one are used selection problems may occur.

FIG. 9 shows differences of signal recordings of seven dot electrodes(S1-S7) of a multi-dot electrode unit where the synthetic referencesignal (SR) calculated according to the present invention is used as areference. The time scale is 0.105-0.130 seconds. The signals S1 to S7are unipolar R-wave signals from the multi-dot electrode unit placed inthe right ventricle of an animal using the indifferent electrode at thehousing as reference. The signal at the bottom is the average of thesignals, i.e. the synthetic reference signal.

These signal differences (S1-SR, . . . , S7-SR) illustrate that there isa low but significant signal amplitude in all signal recordings.

FIG. 10 shows three signal tracings of different processed signals.

The signal at the top show the average of absolute value of differencesignals processed according to the whole processing chain as earlierdescribed according to the invention, i.e. all the signal differencescreated with use of the synthetic reference (S1-SR, . . . , S7-SR) arerectified and then added together. The heart activity is easy to detect,as the pulse amplitude is very distinguished. Detection occurs when thepulse amplitude is higher than a preset detection amplitude threshold(not shown).

The second signal (in the middle) shown in FIG. 10 is included to showthat other signal processing also may be used to get a detectable signalresult from all the electrode signals, in this case the average of theabsolute amplitude values of all unipolar signals. However, the signalprocessing used according to the present invention achieves a moredistinguished result, which has superior detection qualities.

The signal at the bottom of FIG. 10 is the average of the dot electrodepotentials, i.e. the synthetic reference signal (SR).

FIG. 11 shows the same signals as in FIG. 10 but with another time scale(0.105 to 0.130 seconds). As in FIG. 10 the signal at the top in thefigure illustrates the use of a signal processing according to thepresent invention. Investigations have shown that this signal shape hasless variation than the variation in signal shape of each dot electrodesignal, and also less variation than the variations of the averagesignal.

The present invention is not limited to the above-described preferredembodiments. Various alternatives, modifications and equivalents may beused. Therefore, the above embodiments should not be taken as limitingthe scope of the invention, which is defined by the appending claims.

1. Medical system for detecting heart events including an electrode leadprovided with a multi-dot electrode unit (4) that comprises at leastthree dot electrodes (8), said multi-dot electrode unit is adapted to beused for intra-corporal sensing of heart signals, characterized in thatheart signals sensed by each of the dot electrodes are applied to aprocessing means (18, 20, 22, 24) where the signals are combined and asynthetic reference (SR) signal is determined, the differences betweeneach dot electrode heart signal and the synthetic reference voltage aredetermined, and an indication signal is formed based upon saiddifferences, wherein said indication signal is used to detect heartevents.
 2. Medical system according to claim 1, characterized in thatthe synthetic reference signal (SR-signal) is determined according tothe formula:SR-signal=1/N×Σ(U ₁ + . . . +U _(N)), where N is the number of dotelectrodes, and U₁ . . . U_(N) are the dot electrode potentials inrelation to an electrical reference point.
 3. Medical system accordingto claim 2, characterized in that for each dot electrode a differentialdot electrode value A_(diff(i)) is determined by the formula:A _(diff(i)) =U _(i) −SR-signal, where i=1 . . . N.
 4. Medical systemaccording to claim 3, characterized in that said indicating signal isformed by adding together the absolute values of A_(diff(i)), where i=1. . . N.
 5. Medical system according to claim 3, characterized in thatsaid indicating signal is formed by adding together the squared valuesof A_(diff(i)), where i=1 . . . N.
 6. Medical system according to claim3, characterized in that said indicating signal is based upon the signalcontents of A_(diff(i)), where i=1 . . . N.
 7. Medical system accordingto claim 1, characterized in that said indication signal is applied to adiscrimination means adapted to generate an detection signal if theindication signal fulfils predetermined heart event detection criteria.8. Medical system according to claim 1, characterized in that saidprocessing means is arranged in said electrode lead in connection withthe multi-dot electrode unit.
 9. Medical system according to claim 1,characterized in that said multi-dot electrode unit is arranged at thedistal end of said electrode lead.
 10. Medical system according to claim1, characterized in that said synthetic reference signal is determinedas the average value of at least three of the detected dot electrodepotentials.
 11. Medical system according to claim 1, characterized inthat said processing means is arranged in an implantable medical deviceand electrically connected to the multi-dot electrode unit viaconducting means in said electrode lead.
 12. Medical system according toclaim 1, characterized in that multi-dot electrode unit may also be usedto apply stimulation pulses to tissue.
 13. Medical system according toclaim 1, characterized in that said processing means is adapted tochange mode of operation for the multi-dot electrode unit between adetection-mode and a stimulation-mode.